The present invention generally relates to monitor circuits used to detect an abnormal condition in a process or equipment, and more particularly to a monitor circuit in which the sensitivity level is decreased for a period immediately following startup or turnon of the process or equipment so that the monitor circuit does not respond to the transient created by startup or turnon but will respond to an abnormal condition greater than the transient.
It is a common condition in many processes and equipments that a transient condition will be generated upon startup or turnon of the process or equipment. In such processes or equipments wherein conditions are monitored to detect an abnormal condition, this transient condition may have a magnitude which is substantially larger than the steady-state monitored condition limits. While the transient condition may be considered a normal condition incident to the startup or turnon of the process or equipment being monitored, the monitor circuit may nevertheless respond to the transient condition and cause shutdown of the process or equipment. In the past, two techniques have been employed to avoid the erroneous response of monitor circuits to a transient condition produced by startup or turnon of a process or equipment. One technique is to disable the monitor circuit for the period of the transient condition, and the other technique is to slow the response of the monitor circuit to the point where the transient condition will have passed before the monitor circuit can react to it.
While both of the techniques for avoiding erroneous response of monitor circuits to transient conditions incident to startup or turnon are satisfactory to many processes or equipments, the present invention arose in the application of monitor circuits to unattended communications systems. In a specific application, a communications station may employ a microwave transmitter having a travelling wave tube amplifier. When power is first applied to the travelling wave tube, the cathode to helix current may experience a current transient that could be an order of magnitude larger than the normal steady-state operating current. Such a transient is normal and should not result in turnoff of the transmitter due to the erroneous detection of an abnormal condition by the helix overcurrent monitor. On the other hand, it is possible that the travelling wave tube may be turned on into a fault condition which could result in damage to the travelling wave tube occurring during the period of a normal turnon transient condition. Under such conditions, if the monitor circuit is disabled for the period of the turnon transient, there is no protection against a fault until that period has passed. Because only a small amount of energy is required to damage a travelling wave tube, a delay until the transient period has passed cannot be allowed. For the same reason, it is even worse to slow the response time of the monitor circuit to allow the transient to pass unnoticed because the slow response would apply also to all operations of the monitor circuit. Therefore, the monitor circuit must be capable of not only sensing current slightly above the transient level during the period of the turnon transient but it also must be able to respond quickly.